Aerospace components made of superalloys, such as nickel and cobalt-based superalloys, are susceptible to oxidation, which can reduce their service life and necessitate their replacement or repair. For example, gas turbine engine components such as the burner assembly, turbine vanes, nozzles, and blades are susceptible to oxidation because they encounter severe, high temperature conditions. As used herein, “severe operating conditions” include high gas velocities and exposure to salt, sulfur, and sand, which can cause hot corrosion or erosion. As further used herein, “high temperature conditions” refers to temperatures of about 700° C. to about 1150° C. The oxidation resistance of such superalloy components can be enhanced by applying protective coatings.
Simple aluminide coatings are used on superalloy components to improve oxidation resistance, especially when the cost of production is a consideration. Platinum aluminide coatings are used in even more demanding applications. There are several drawbacks to conventional aluminum deposition techniques. For example, chemical vapor deposition (CVD) is costly and requires the use of dangerous gases. In another example, the use of pack cementation is less costly, but there are also drawbacks associated with this conventional deposition technique, such as the introduction of impurities into the aluminum, thereby reducing coating life. For both of these gaseous aluminizing processes, the temperatures used are so high that the aluminum diffuses into the superalloy substrate/component as the coating is deposited—the resultant surface aluminide is only about 20-30% aluminum. There are also lower temperature aluminum CVD deposition processes that do not result in aluminum diffusion, but these processes are only used in a few specialized applications, because of the dangerous gases involved.
In addition, as CVD and pack cementation deposition processes are performed at high temperatures and under aggressive deposition conditions, high cost masking techniques prior to deposition are used to ensure that high stress areas of the superalloy component are not coated. After deposition or coating, the masks are removed. High temperature (and high cost) masking techniques include applying masking pastes to the component by spraying or dipping. Extreme care (and labor) must be taken to ensure that only the desired areas are coated. These pastes form hard deposits that are difficult and labor intensive to remove.
Aluminum electroplating processes may also be used to deposit aluminum at high purity levels, but conventional aluminum electroplating is complex, costly, performed at high temperatures, and/or requires the use of flammable solvents and pyrophoric compounds, which decompose, evaporate, and are oxygen-sensitive, necessitating costly specialized equipment and presenting serious safety and environmental challenges to a commercial production facility. In addition, for all aluminum electroplating processes on superalloys, the aluminum is present after plating merely as an aluminum layer on the surface of the substrate. The aluminum layer thereafter needs to be bonded and diffused into the superalloy component to produce the desired high temperature oxidation resistant aluminide coating. As used herein, the term “aluminide coating” refers to the coating after diffusion of aluminum into the superalloy component. If conventional aluminum diffusion temperatures of about 1050° C. to about 1100° C. are used, undesirable microstructures are created. The use of flammable and dangerous liquids during the electroplating of aluminum have been avoided when plating steel etc., that is non-superalloy substrates for non-aerospace applications, by using Ionic liquids. The process includes a first pretreatment step in which the substrate is cleaned and degreased, and in which oxides are removed through acid treatment (commonly referred to as “pickling”) or through wet blast abrasion. The substrate is thereafter dried. In the second step, the metal substrate is electroplated using the ionic liquid at a temperature ranging from about 60° C. to about 100° C. In addition the ionic liquids do not involve flammable solvents or pyrophoric compounds.
It is well established that small additions of certain so-called “reactive elements” including silicon, hafnium, zirconium, cerium, and lanthanum increase the oxidation resistance of high temperature aluminide coatings. Unfortunately, the co-deposition of aluminum and a reactive element is difficult, expensive, and can be dangerous. In a best case scenario, the co-deposit requires at least two separate deposition processes, such as the initial deposit of aluminum by a chemical vapor deposition process, pack cementation process, or the like, followed by deposition of the reactive element by another chemical vapor deposition process in the same or a different reactor. In one example, a heat-treated slurry coating containing aluminum and hafnium particles has been used in an attempt to co-deposit aluminum and hafnium to form a protective aluminide-hafnium coating, but the results have been disappointing with the hafnium particles not sufficiently diffusing into the aluminum, the base metal of the coated component oxidizing, and the concentration of the reactive element unable to be controlled.
A particular form of aluminide/reactive element coating that has been well established in the art for use in high temperature coatings is a derivative family of alloys described as “MCrAlXs”. MCrAlXs are useful because they exhibit excellent resistance to oxidation and hot corrosion. These alloys, where the “M” represents a metal that may be either iron (Fe), nickel (Ni), or cobalt (Co), or alloys thereof such as iron-base alloys, nickel-base alloys and cobalt-base alloys, and where “X” represents a reactive element that may be Y, Hf, Zr, Si, Ta, Ti, Nb, Mo, W, La, or other reactive element. A common form of MCrAlX coatings are the MCrAlYs, where the “X” reactive element is specified as Yttrium. Further, with the “M” metal specified, these particular coatings are generically referred to as FeCrAlXs, NiCrAlXs, or CoCrAlXs, respectively. MCrAlX coatings are applied to superalloy substrates using expensive techniques, including for example vapor deposition, plasma-based techniques, and high-velocity spraying, among others. Further, some attempts have been made to co-deposit CrAlX powders, particularly CrAlY powders, suspended in aqueous nickel or cobalt electroplating baths, but performance was inferior due to uniformity and impurity issues.
Accordingly, it is desirable to provide methods for producing a high purity, high temperature oxidation and hot corrosion resistant coating on superalloy components, including gas turbine engine components. In addition, it is desirable to provide methods for producing a high temperature oxidation and hot corrosion resistant MCrAlX coating on a superalloy component using a simplified, lower cost, safe, and environmentally-friendly method including the use of low temperature masking techniques. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.